1. Field of the Invention
The present invention relates to an impulse-based communication system and, more particularly, to a synchronization acquisition method for an impulse-based communication system such as a UWB (Ultra Wide Band) system.
2. Description of the Related Art
UWB systems are systems that transmit and receive signals in the form of pulses, without using a carrier, and were approved in 2002 by the FCC for commercial use. According to the definition by the FCC, UWB is “any radio signal that occupies a bandwidth equal to or greater than 20% of the center frequency or that occupies a bandwidth equal to or greater than 500 MHz.” UWB has been attracting considerable attention as technology that can achieve high-speed data communications. UWB has the advantage of being able to share the spectrum with other communication systems because UWB signals are transmitted using a very wide bandwidth and at low power. Further, as signals are transmitted as pulses, problems associated with the effects of multipath and fading can also be solved (refer to Non-Patent Documents 1 and 2).
Short-pulse UWB waveforms are used for indoor communications because the effects of multipath interference can be avoided. In the description hereinafter given, an additive white Gaussian noise AWGN model free from multipath is assumed. As described in Non-Patent Document 5, a monocycle waveform w(t) is given by the following equation (1).
                              w          ⁡                      (            t            )                          =                              {                          1              -                              4                ⁢                                                      π                    ⁡                                          (                                              t                                                  τ                          m                                                                    )                                                        2                                                      }                    ⁢          exp          ⁢                      {                                          -                2                            ⁢                                                π                  ⁡                                      (                                          t                                              τ                        m                                                              )                                                  2                                      }                                              (        1        )            where τm is the magnitude of the pulse width.
FIG. 9 is a diagram showing one example of the waveform of a transmit signal modulated by DS (Direct Sequence). In the example shown, one bit of information is represented by S successive pulses (S is a positive integer) in a pulse train of period Tf. Other than the DS shown in FIG. 9 which involves pulse phase reversal, TH-UWB (Time-Hopping UWB) is known for use as the modulation scheme.
FIG. 10 is a diagram for explaining the DS-UWB modulation scheme. As shown, in DS-UWB the phase of each pulse in the pulse train is reversed to represent a 0 or a 1. This is similar to the BPSK (Binary Phase Shift Keying) technique employed in traditional CDMA, etc. in which the phase of the carrier is not reversed or reversed to represent information 1 or 0.
FIG. 11 is a diagram for explaining the TH-UWB modulation scheme. As shown, in TH-UWB the position of each pulse on the time axis is changed, for example, by 125 picoseconds, and a 0 or a 1 is represented by the position thus changed.
In either modulation scheme, one bit of information is represented by spectrum spreading with S successive pulses (S is a positive integer) (PN code or Baker code) as earlier described.
In UWB systems employing the DS modulation or TH modulation, code synchronization is an important issue. In many cases, synchronization acquisition must be performed in an environment where the SN ratio is very low or in the presence of interfering waves.
For spread spectrum communications performed by UWB systems employing DS modulation or TH modulation, the prior art provides two synchronization acquisition methods, a matched-filter-based synchronization acquisition method and a correlator-based synchronization acquisition method.
The matched-filter-based synchronization acquisition method can achieve quick synchronization acquisition, but requires a large amount of hardware.
On the other hand, the correlator-based synchronization acquisition method can be implemented using relatively simple hardware, but requires a longer time to achieve synchronization acquisition.
To reduce the synchronization acquisition time, a synchronization acquisition method using a plurality of correlators is proposed, but this method adds complexity to the receiver design and increases power consumption.
In one conventional initial synchronization method that uses the correlation method, the phase state is sequentially changed from one possible state to another until the correct code phase is obtained. Each phase is evaluated for correctness by applying despreading to the received signal and checking the result. If the estimated code phase is correct, despreading is performed and a correlation peak value is detected. If the estimated code phase is not correct, despreading is not performed and the reference signal steps to a new phase for the next estimation. This technique is called the serial search (refer to Non-Patent Document 3).
However, this correlator-based serial search method has the problem that, as the period that contains no information, in the transmitted signal, becomes longer, it takes a longer time to achieve synchronization acquisition at the receiving end.
In view of the above prior art problem, and to reduce the synchronization acquisition time required at the receiving end, the applicant of the present invention proposed a novel synchronization acquisition method in Japanese Patent Application No. 2003-379800 filed on Nov. 10, 2003. In this synchronization acquisition method, which is proposed for impulse-based communication, a pulse detecting signal shifted in phase by a predetermined amount of time with respect to the transmit information of a pulse signal having a prescribed period is generated at the receiving end as well as at the transmitting end, and synchronization is established at the receiving end by using this pulse detecting signal; then, code synchronization is established by shifting the phase by the predetermined amount of time with respect to the synchronized pulse detecting signal.
According to the above prior art, the synchronization acquisition time can be reduced, because synchronization acquisition can be achieved quickly at the receiving end even when no transmit information is contained in the transmitted signal.
However, the above prior art has had the problem that, in an environment where multipath is likely to occur along the propagation path, the pulse detecting signal also becomes complex at the receiving end because of the effects of multipath and it becomes necessary to repeat the synchronization acquisition, thus making the synchronization acquisition time correspondingly longer. There has also been the problem that, after the code synchronization is established, if a pulse detecting signal having a greater electric field strength is detected in a multipath environment, the synchronization acquisition has to be performed once again by suspending the demodulation operation and switching to the pulse detection operation. These problems will be described in further detail with reference to FIGS. 12A and 12B.
FIG. 12A shows one transmitted pulse, and FIG. 12B shows the received signal made complex due to the effects of multipath when the transmitted signal show in part FIG. 12A was received. To acquire synchronization for such a signal, the prior art has had to perform the synchronization acquisition operation repeatedly, because it is not known which peak in the received signal corresponds to the peak of the original signal and it thus becomes difficult to receive the original signal with reduced noise. Further, each time a better peak is detected after establishing the code synchronization, the synchronization has had to be acquired once again by suspending the demodulation operation temporarily and switching to the synchronization establishing operation performed using the pulse detecting signal. Following documents are prior arts of the present invention.
1. Japanese Patent Application No. 2003-379800
2. M. Z. Win, R. A. Scholtz, “Ultra-Wide Bandwidth Time-Hopping Spread-Spectrum Impulse Radio for Wireless Multiple-Access Communications” IEEE Trans. On Commun., vol. 48, no. 4, April 2002
3. K. Siwiak, P. Withington, S. Phelan, “Ultra-Wide Band Radio: The Emergence of an Important RF Technology,” Vehicular Technology Conference, VTC 2001 Spring, IEEE VTS 53rd, Vol. 2, pp. 1169-1172, May 2001
4. Roger L. Peterson, Roger L. Ziemer, David E. Borth, “Introduction to Spread Spectrum Communications” Prentice Hall, 1995
5. Kazimierz Siwiak, “Ultra-Wide Band Radio: Introducing a New Technology,” Vehicular Technology Conference, VTC 2001 Spring, IEEE VTS 53rd, Volume; 2, 6-9 May 2001
6. Femando Ramirez-Mireles, “On the Performance of Ultra-Wide-Band Signal in Gaussian Noise and Dense Multipath,” IEEE Trans. on Vehicular Technology, vol. 50, no. 1, January 2001
7. Jack K. Holmes, “Acquisition Time Performance of PN Spread-Spectrum Systems,” IEEE Trans. Commun., COM-25, 8, pp. 778-783 (August 1977)